FIG. 1 illustrates the basic features in the active area of a conventional color flat-panel CRT display that operates according to field-emission principles. The field-emission display ("FED") in FIG. 1 consists of an electron-emitting device and a light-emitting device. The electron-emitting device, commonly referred to as a cathode, contains electron-emissive elements 1 that emit electrons over a wide area. The emitted electrons are directed towards light-emissive elements 2 distributed over a corresponding area in the light-emitting device. Upon being struck by the electrons, light-emissive elements 2 emit light that produces an image on the viewing surface of the FED.
Specifically, electron-emissive elements 1 are situated over emitter electrodes 3, one of which is shown in FIG. 1. Control electrodes 4 cross over, and are electrically insulated from, emitter electrodes 3. A set of electron-emissive elements 1 are electrically coupled to each emitter electrode 3 where it is crossed by a control electrode 4. For simplicity, FIG. 1 depicts only one electron-emissive element 1 at each electrode crossing location. When a suitable voltage is applied between a control electrode 4 and an emitter electrode 3, that control electrode 4 extracts electrons from associated electron-emissive element 1. An anode (not shown) in the light-emitting device attracts the electrons to light-emissive elements 2 laterally separated by black matrix 5 over transparent faceplate 6.
Electron emission from a single electron-emissive element 1 under the control of associated control electrode 4 is generally distributed throughout a solid cone with a maximum half angle greater than 45.degree. relative to the vertical in FIG. 1. For reference purposes, FIG. 1 illustrates a 45.degree.-half angle cone at the tip of one electron-emissive element 1. At the light-emitting device, undeflected electrons are distributed over an area generally represented by item 7 in FIG. 1. Area 7 increases as the distance between the cathode and anode structures increases. As FIG. 1 illustrates, undeflected electrons emitted by one electron-emissive element 1 can strike area outside intended light-emissive element 2.
FEDs that operate at high anode voltages for improved brightness and lifetime require comparatively large cathode-to-anode spacings in order to avoid electrical arcing between components of the anode and cathode structures. The potentialities of having electrons strike undesired places, e.g., light-emissive elements 2 adjacent to intended light-emissive element 2, are therefore of special concern for FEDs operating with high anode voltages.
The electron-emitting device in an FED commonly contains a focusing system that helps control the trajectories of the electrons so that they largely only strike the intended light-emissive elements. The focusing system normally extends above the control electrodes. The lateral relationship of the focusing system to the sets of electron-emissive elements is critical to achieving high display performance.
FIGS. 2a-2c illustrate a conventional variation of the FED of FIG. 1 to which a focusing system 8 has been added. Focusing system 8 locally deforms the electric field existing between the anode and cathode structures to form an electron lens that alters the electron trajectories. The amount of change in the electron trajectories depends on factors such as the initial trajectories, the strength of the electron lens, and the times of flight within the lens. Ideally, the characteristics of focusing system 8 are chosen in such a way that substantially all impinging electrons strike intended electron-emissive element 2 as indicated in FIG. 2a. However, the electrons often strike undesired areas when the electron lens is underfocused as shown in FIG. 2b or overfocused as shown in FIG. 2c.
The ability of the electron lens to properly focus the emitted electrons depends on the physical characteristics of the focusing system. Generally, the focusing system needs to be capable of maintaining a desired potential. U.S. Pat. No. 5,528,103 illustrates various configurations for an electron focusing system that can maintain a potential in an FED. Unfortunately, all of the focusing systems in U.S. Pat. No. 5,528,103 either provide insufficient focusing capability or raise concerns with respect to electrical short circuiting to the control electrodes.
It is desirable to have a focusing system that provides a suitable amount of electron focusing for an electron-emitting device without running any significant reliability or other short-circuiting risk. Specifically, the potential that controls the electron trajectories should be provided to the focusing system in a manner that avoids reliability concerns. It is also desirable to have a technique for readily fabricating such a focusing system.